Technique for chemical mechanical polishing silicon

ABSTRACT

Chemical mechanical polishing of a silicon layer, such as a polycrystalline silicon, is improved by initially chemical mechanically polishing the silicon layer with an oxide-polishing slurry. Then the silicon layer is chemical mechanically polished with a silicon-polishing slurry until the substrate is planarized.

BACKGROUND

[0001] The present invention relates generally to chemical mechanicalpolishing of substrates, and more particularly to a technique forchemical mechanical polishing silicon.

[0002] An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. After each layer is deposited, it may be etched tocreate circuitry features. One fabrication step involves the formationof vias, plugs and lines to provide conductive paths between thin filmcircuits. Vias can be created by depositing a conductive layer, such aspolycrystalline silicon (polysilicon or p-Si), over a patternedinsulative layer, and then planarizing the polysilicon layer bypolishing or etching until the insulative layer is exposed. The portionsof the polysilicon layer remaining between the raised pattern of theinsulative layer form the vias, plugs and lines.

[0003] Chemical mechanical polishing (CMP) is one accepted method ofplanarizing and exposing the insulative layer. This method typicallyrequires that the substrate be mounted on a carrier or polishing head.The exposed surface of the substrate is placed against a moving, e.g.,rotating, polishing pad. The polishing pad may be either a “standard”pad or a fixed-abrasive pad. A standard pad has a durable roughenedsurface, whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load, i.e.,pressure, on the substrate to push it against the polishing pad. Apolishing slurry, including at least one chemically-reactive agent, andabrasive particles if a standard pad is used, or a chemical solutionwithout abrasive particles if a fixed abrasive pad is used, is suppliedto the surface of the polishing pad. An effective CMP process not onlyprovides a high polishing rate, but also provides a substrate surfacewhich is finished (lacks small-scale roughness) and flat (lackslarge-scale topography). In addition, an effective CMP process producesminimal dishing (over-polishing of the polysilicon layer so that it islower than the insulative layer) and erosion (removal of the insulativelayer).

[0004] One problem that has been encountered in CMP, particularly in thepolishing of polysilicon layers, is non-uniform polishing, such as theso-called “center slow effect”, which is the tendency for the substratecenter to be polished more slowly than the substrate edge. The centerslow effect typically results in underpolishing (the removal of toolittle material) at the central portion of the substrate. Equivalently,if the polishing parameters are changed to increase the amount ofmaterial removed from the substrate central portion, then the outerportion of the substrate will be overpolished, resulting in dishing anderosion. The underpolishing of the substrate center or the overpolishingof the substrate edge reduces the overall flatness of the substrate,making either the center or the edge of the substrate unsuitable forintegrated circuit fabrication and reducing process yield.

SUMMARY

[0005] In one aspect, the invention is directed to a method of polishinga silicon layer on a substrate. In the method, the silicon layer ischemical mechanically polished with an oxide-polishing slurry to removeany native oxides, and chemical mechanically polished with asilicon-polishing slurry.

[0006] Implementations of the invention may include the following. Thesilicon layer may be polished until it is planarized or until anunderlying layer is exposed. The first CMP step steps before the siliconlayer is substantially planarized or the underlying layer is exposed.The first CMP step and a portion of the second CMP step may be performedat the same polishing station. The second CMP step may be performed at aplurality of polishing stations. The oxide-polishing slurry may bedistributed onto a polishing pad at the polishing station before, as, orafter the substrate is brought into contact with the polishing pad. Inaddition, distribution of the oxide-polishing slurry may stop before,as, or after distribution of the silicon-polishing slurry starts. Thesubstrate may be buffed with a buffing solution after the second CMPstep. The silicon layer may be polycrystalline silicon, amorphoussilicon, or single crystalline silicon, and the silicon-polishing slurrymay be a polycrystalline silicon-polishing slurry, an amorphoussilicon-polishing slurry, or a single crystalline silicon-polishingslurry, respectively. The silicon-polishing slurry may have aselectivity of at least 1:50. The first CMP step may less than about tenseconds, e.g., about three to five seconds.

[0007] In another aspect, the invention is directed to a method ofplanarizing a substrate. In the method, a silicon layer is formed on anon-planar surface of the substrate, an oxide-polishing slurry issupplied to a surface of a polishing pad, and the substrate is broughtinto contact with the polishing pad. The substrate is briefly chemicalmechanically polished with the oxide-polishing slurry to remove a nativeoxide layer from a surface of the silicon layer. After the substratecontacts the polishing pad, a silicon-polishing slurry is supplied tothe polishing pad, and the substrate is chemical mechanically polishedwith the silicon-polishing slurry until the silicon layer issubstantially planarized.

[0008] Advantages of the invention may include the following. The centerslow effect is reduced, improving the flatness and finish of thesubstrate, reducing dishing and erosion, and increasing the processyield.

[0009] Other features and advantages will be apparent from the followingdescription, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic exploded perspective view of a chemicalmechanical polishing apparat

[0011]FIGS. 2A-2I are schematic cross-sectional views of a substrateillustrating techniques of the present invention.

DETAILED DESCRIPTION

[0012] Referring to FIG. 1, one or more substrates 10 will be polishedby a chemical mechanical polishing (CMP) apparatus 20. A description ofa similar polishing apparatus may be found in U.S. Pat. No. 5,738,574,the entire disclosure of which is incorporated herein by reference.Polishing apparatus 20 includes a lower machine base 22 with a table top23 mounted thereon and a removable outer cover (not shown). Table top 23supports a series of polishing stations, including a first polishingstation 25 a, a second polishing station 25 b, a final polishing station25 c, and a transfer station 27. Transfer station 27 forms a generallysquare arrangement with polishing stations 25 a, 25 b and 25 c. Transferstation 27 serves multiple functions, including receiving individualsubstrates 10 from a loading apparatus (not shown), washing thesubstrates, loading the substrates into the carrier heads, receiving thesubstrates from the carrier heads, washing the substrates again, andfinally, transferring the substrates back to the loading apparatus.

[0013] Each polishing station includes a rotatable platen 30 on which isplaced a polishing pad. If substrate 10 is an “eight-inch” (200millimeter) or “twelve-inch” (300 millimeter) diameter disk, then theplaten and polishing pad will be about twenty inches or thirty inches indiameter, respectively. Each platen 30 may be a rotatable aluminum orstainless steel plate connected to a platen drive motor (not shown). Formost polishing processes, the platen drive motor rotates platen 30 atthirty to two hundred revolutions per minute, although lower or higherrotational speeds may be used.

[0014] At first and second polishing stations 25 a and 25 b, platen 30may support a two-layer polishing pad 100 having an inner layer 104 andan outer layer 106 with a rough surface 102 (see FIG. 2B). The innerlayer of the pad may be attached to platen 30 by a pressure-sensitiveadhesive layer. The outer layer may be harder than the inner layer. Forexample, the outer layer may be composed of microporous polyurethane orpolyurethane mixed with a filler, whereas the inner layer may becomposed of compressed felt fibers leached with urethane. A two-layerpolishing pad, with the outer layer composed of IC-1000 or IC-1400 andthe inner layer composed of SUBA-4, is available from Rodel, Inc. ofNewark, Del. (IC-1000, IC-1400 and SUBA-4 are product names of Rodel,Inc.).

[0015] At final polishing station 25 c, platen 30 may support apolishing pad 110 having a generally smooth surface 112 and a singlesoft layer 114 (see FIG. 2F). This single layer pad may be attached toplaten 30 by a pressure-sensitive adhesive layer. The soft pad may becomposed of a napped poromeric synthetic material. A suitable softpolishing pad is available from Rodel, Inc., under the trade namePolitex. The polishing pads may be embossed or stamped with a pattern toimprove distribution of slurry across the face of the substrate.

[0016] Each polishing station 25 a, 25 b and 25 c also includes acombined slurry/rinse arm 52 a, 52 b and 52 c, respectively, thatprojects over the associated polishing pad surface. Each slurry/rinsearm 52 a-52 c may include two or more slurry supply tubes 54 a and 54 bconnected to a slurry delivery system to provide two or more slurries tothe surface of the polishing pad. Typically, sufficient slurry isprovided to cover and wet the entire polishing pad. Each slurry/rinsearm 52 a-52 c also includes several spray nozzles (not shown) whichprovide a high-pressure rinse of the polishing pad at the end of eachpolishing and conditioning cycle.

[0017] Two or more intermediate washing stations 56 a and 56 b may bepositioned between neighboring polishing stations 25 a, 25 b and 25 c.The washing stations rinse the substrates as they pass from onepolishing station to another.

[0018] Each polishing station 25 a-25 c may further include anassociated pad conditioner apparatus 40. Each pad conditioner apparatus40 has a rotatable arm 42 holding an independently-rotating conditionerhead 44 and an associated washing basin 46. The pad conditionerapparatus maintains the condition of the polishing pad so that it willeffectively polish substrates.

[0019] A rotatable multi-head carousel 60 is positioned above lowermachine base 22. Carousel 60 is supported by a center post 62 and isrotated thereon about a carousel axis 64 by a carousel motor assemblylocated within machine base 22. Center post 62 supports a carouselsupport plate 66 and a cover 68. Carousel 60 includes four carrier headsystems 70 a, 70 b, 70 c, and 70 d. Three of the carrier head systemsreceive and hold substrates, and polish them by pressing them againstthe polishing pads on the platens of polishing stations 25 a-25 c. Oneof the carrier head systems receives a substrate from and delivers asubstrate to transfer station 27.

[0020] The four carrier head systems 70 a-70 d are mounted on carouselsupport plate 66 at equal angular intervals about carousel axis 64.Center post 62 allows the carousel motor to rotate carousel supportplate 66 and to orbit carrier head systems 70 a-70 d and the substratesattached thereto about carousel axis 64.

[0021] Each carrier head system 70 a-70 d includes a carrier or carrierhead 80. A carrier drive shaft 74 connects a carrier head rotation motor76 (shown by the removal of one quarter of cover 68) to carrier head 80so that each carrier head 80 can independently rotate about its ownaxis. There is one carrier drive shaft and motor for each head. Inaddition, each carrier head 80 independently laterally oscillates in aradial slot 72 formed in carousel support plate 66. A slider (not shown)supports each drive shaft in its associated radial slot. A radial drivemotor (not shown) may move the slider to laterally oscillate the carrierhead.

[0022] The carrier head 80 performs several mechanical functions.Generally, the carrier head holds the substrate against the polishingpad, evenly distributes a downward pressure across the back surface ofthe substrate, transfers torque from the drive shaft to the substrate,and ensures that the substrate does not slip out from beneath thecarrier head during polishing operations.

[0023] The carrier head 80 may include a flexible membrane (not shown)which provides a substrate receiving surface. A description of asuitable carrier head 80 may be found in U.S. patent application Ser.No. 08/745,679, entitled a CARRIER HEAD WITH a FLEXIBLE MEMBRANE FOR aCHEMICAL MECHANICAL POLISHING SYSTEM, filed Nov. 8, 1996, by Steven M.Zuniga et al., assigned to the assignee of the present invention, theentire disclosure of which is incorporated herein by reference.

[0024]FIGS. 2A-2G illustrate a process of chemical-mechanicallypolishing a silicon layer. As shown in FIG. 2A, substrate 10 includes aninsulative layer 14, such as silicon dioxide, disposed on asemiconductive layer, such as a silicon wafer 12. The insulative layer14 is either patterned or disposed on a patterned underlying layer sothat it provides a non-planar outer surface. A polysilicon layer 16 isdisposed over insulative layer 14. As shown, the outer surface ofpolysilicon layer 16 almost exactly replicates the underlying structuresof layer 14, creating a series of peaks and valleys so that the exposedsurface of the substrate is non-planar. In other embodiments, othersorts of silicon layers, such as amorphous silicon (a-Si) and singlecrystal silicon, may be used instead of a polysilicon layer.

[0025] As discussed above, one purpose of planarization is to polishpolysilicon layer 16 until the top surface of insulative layer 14 isexposed, thus leaving polysilicon vias between the insulative islands,and polysilicon plugs in any holes in the insulative layer (see FIG.2G). Unfortunately, as previously noted, one problem with polishingpolysilicon is the center slow effect. Without being limited to anyparticular theory, one possible cause of the center slow effect is theformation of a layer of native oxide 18 on the outer surface ofpolysilicon layer 16 when the substrate is exposed to the atmosphere.Although this native oxide layer may be extremely thin, e.g., on theorder of a few atomic layers, polysilicon-polishing slurries are highlyselective (i.e., the polishing rate depends strongly on the compositionof the surface being polished) to avoid polishing of underlyinginsulative layer 14. Consequently, native oxide layer 18 interferes withthe polishing of the polysilicon layer 16. Specifically, the polishingparameters selected for uniform polishing of polysilicon may result innon-uniform polishing of the native oxide layer. The non-uniformpolishing of native oxide layer 18 leads to non-uniform polishing ofpolysilicon layer 16. In fact, despite the high selectivity of thepolysilicon-polishing slurry, the non-uniformity may be sufficientlysevere that dishing and erosion results.

[0026] Referring to FIG. 2B, substrate 10 is polished at polishingstation 25 a using a standard hard polishing pad 100. Initially,oxide-polishing slurry 50 a is distributed to the polishing pad surfacevia slurry supply tube 54 a. The oxide-polishing slurry may be suppliedto the polishing pad surface before, as, and/or after the substrate“touches down” on the pad.

[0027] The oxide-polishing slurry may include deionized water, fumedsilica abrasive particles, and a chemically reactive agent, such aspotassium hydroxide (KOH), which may also adjust the pH of the slurry.Alternately, KOH may be replaced with an amine-based chemically reactiveagent if mobile ions from the KOH contaminate the polysilicon layer. Asuitable oxide-polishing slurry is available from Cabot Corp., ofAurora, Ill., under the trade name SS-12.

[0028] However, as represented by FIG. 2C, almost immediately after thesubstrate has been brought into contact with the polishing pad, e.g.,after less than about ten seconds of polishing, the CMP apparatus startsdistributing polysilicon-polishing slurry 50 b to the polishing padsurface via slurry supply tube 54 b. In one implementation, thepolysilicon-polishing slurry could be distributed after about just threeto five seconds of polishing.

[0029] The oxide polishing may occur only during the “ramp-up” period atthe start of the polishing operation during which the platen is broughtup to the desired rotational speed and the carrier head is pressurizedto provide the desired load. Since the native oxide layer is very thin,oxide polishing can occur at lower rotational speeds and pressures thanpolysilicon polishing.

[0030] The beginning of the polysilicon-polishing slurry distributionshould generally coincide with the ending of the distribution of theoxide-polishing slurry. Specifically, distribution of thepolysilicon-polishing slurry may start exactly as distribution of theoxide-polishing slurry stops. Alternately, distribution of thepolysilicon-polishing slurry may begin just after distribution of theoxide-polishing slurry ends, so that no slurry is distributed to thepolishing pad for a brief period, e.g., less than about one second.Alternately, distribution of the polysilicon-polishing slurry may beginjust before distribution of the oxide-polishing slurry ends, so that thetwo slurries are provided to the polishing pad simultaneously for abrief period, e.g., less than about two or three seconds.

[0031] The polysilicon-polishing slurry may include deionized water,formed silica abrasive particles, and an amine-based chemically reactiveagent which may also adjust the pH of the slurry. Suitablepolysilicon-polishing slurries include EPP-1000, EPP-1060, andEPP-1000LRP, available from Cabot Corp.; Planarlite-6101,Planarlite-6102 and Planarlite-6203, available from Fujimi, ofWilsonville, Oreg.; and SDE-3000 available from Rodel, Inc. Both theoxide-polishing slurry and the polysilicon-polishing slurry may includeother additives to provide selective polishing of their respectivematerials. Polysilicon-polishing slurries normally have a selectivity ofat least 1:100, the polishing rate of polysilicon is one-hundred timesthe polishing rate of a reference material (typically a thermallydeposited oxide). The polysilicon-polishing slurries mentioned abovefrom Cabot, Fujimi and Rodel have selectivities of about 1:100 to1:1000, depending on the composition of the insulative layer.Preferably, polysilicon-polishing slurry 50 b has a selectivity of atleast 1:50 to substantially prevent polishing of the underlyinginsulative layer.

[0032] As shown by FIGS. 2B and 2C, the brief polishing at polishingstation 25 a with oxide-polishing slurry 50 a is sufficient to removenative oxide layer 18 from the substrate surface.

[0033] Referring to FIGS. 2C and 2D, once the native oxide layer hasbeen removed, the substrate is polished with polysilicon-polishingslurry 50 b. This polishing continues until polysilicon layer 16 ispartially or substantially planarized, i.e., the large scale topographysuch as the peaks and valleys have been substantially removed.

[0034] Referring to FIGS. 2D and 2E, when about half of the polysiliconlayer has been removed, the substrate may be transported to polishingstation 25 b, and the planarization process may be completed using asecond standard hard polishing pad 100 and a secondpolysilicon-polishing slurry 50 c. The polysilicon layer is polished atpolishing station 25 b until the underlying patterned insulative layer14 is exposed.

[0035] The polysilicon-polishing slurry used at polishing station 25 bmay be substantially the same as the polysilicon-polishing slurry usedat polishing station 25 a. To prevent contamination of the secondpolysilicon-polishing slurry, the substrate may be washed at washingstation 56 a to remove any oxide polishing-slurry before it istransported to polishing station 25 b. In any event, sincepolysilicon-polishing slurry is used exclusively at polishing station 25b, insulative layer 14 should not be polished after it is exposed.

[0036] Referring to FIG. 2F, the substrate is transported to polishingstation 25 c where it is buffed using a standard soft polishing pad 110and a buffing solution 50 d, e.g., deionized water. Buffing solution 50d is supplied to the polishing pad at polishing station 25 c byslurry/rinse arm 52 c. Referring to FIG. 2G, the resulting substrate 10has a flat and finished outer surface 19 in which the underlyingpatterned insulative layer has been completely exposed.

[0037] Alternately, referring to FIG. 2H, a thin residual layer 17having a thickness T may remain over the insulative layer afterpolishing at polishing station 25 b. In this case, as can bee seen fromFIG. 2I, the final buffing process at polishing station 25 c alsoremoves the residual layer. In addition, buffing solution 50 d mayinclude abrasive particles.

[0038] The use of the oxide-polishing slurry to remove the native oxidelayer at the beginning of the polishing process, followed quickly by theuse of the highly selective polysilicon-polishing slurry to polish thepolysilicon layer, substantially reduces polishing non-uniformity,dishing and erosion. In addition, it maintains a high polishing rate andavoids polishing of the underlying patterned insulative layer.

[0039] Although the planarization of a polysilicon layer has beendescribed, the invention may also be applicable to planarization ofother layers, particularly silicon layers, on which a native oxide layermay be formed, such as amorphous silicon (a-Si) and single crystalsilicon. The polysilicon-polishing slurry would be replaced by anamorphous silicon-polishing slurry or a single crystal silicon-polishingslurry.

[0040] The invention is not limited to the embodiment depicted anddescribed. Rather, the scope of the invention is defined by the appendedclaims.

What is claimed is:
 1. A method of polishing a silicon layer on asubstrate, comprising: (a) chemical mechanically polishing the siliconlayer with an oxide-polishing slurry to remove any native oxides; and(b) chemical mechanically polishing the silicon layer with asilicon-polishing slurry.
 2. The method of claim 1 , wherein the siliconlayer is polished until an underlying layer is exposed.
 3. The method ofclaim 1 , wherein the silicon layer is polished until it is planarized.4. The method of claim 1 , wherein step (a) and at least a portion ofstep (b) are performed at the same polishing station.
 5. The method ofclaim 4 , wherein step (b) comprises distributing the oxide-polishingslurry onto a polishing pad at the polishing station before thesubstrate is brought into contact with the polishing pad.
 6. The methodof claim 4 , wherein step (b) comprises distributing the oxide-polishingslurry onto a polishing pad at the polishing station as the substrate isbrought into contact with the polishing pad.
 7. The method of claim 4 ,wherein step (b) comprises distributing the oxide-polishing slurry ontoa polishing pad at the polishing station after the substrate is broughtinto contact with the polishing pad.
 8. The method of claim 1 , whereindistribution of the oxide-polishing slurry is stopped beforedistribution of the silicon-polishing slurry starts.
 9. The method ofclaim 1 , wherein distribution of the oxide-polishing slurry is stoppedas distribution of the silicon-polishing slurry starts.
 10. The methodof claim 1 , wherein distribution of the oxide-polishing slurry isstopped after distribution of the silicon-polishing slurry starts. 11.The method of claim 1 , wherein step (b) is performed at a plurality ofpolishing stations.
 12. The method of claim 1 , further comprisingbuffing the substrate with a buffing solution after step (b).
 13. Themethod of claim 1 , wherein the silicon layer is polycrystallinesilicon.
 14. The method of claim 13 , wherein the silicon-polishingslurry is a polycrystalline silicon-polishing slurry.
 15. The method ofclaim 1 , wherein the silicon layer is amorphous silicon.
 16. The methodof claim 15 , wherein the silicon-polishing slurry is an amorphoussilicon-polishing slurry.
 17. The method of claim 1 , wherein thesilicon layer is single crystalline silicon.
 18. The method of claim 17, wherein the silicon-polishing slurry is a single crystallinesilicon-polishing slurry.
 19. The method of claim 1 , wherein thesilicon-polishing slurry has a selectivity of at least 1:50.
 20. Themethod of claim 1 , wherein step (a) lasts for less than about tenseconds.
 21. The method of claim 20 , wherein step (a) lasts for aboutthree to five seconds.
 22. A method of planarizing a substrate,comprising: (a) forming a silicon layer on a non-planar surface of thesubstrate; (b) supplying an oxide-polishing slurry to a surface of apolishing pad; (c) bringing the substrate into contact with thepolishing pad; (d) briefly chemical mechanical polishing the substratewith the oxide-polishing slurry to remove a native oxide layer from asurface of the silicon layer; (e) after the substrate contacts thepolishing pad, supplying a silicon-polishing slurry to the polishingpad; and (f) chemical mechanical polishing the substrate with thesilicon-polishing slurry until the silicon layer is substantiallyplanarized.
 23. The method of claim 22 , wherein the silicon layer ispolycrystalline silicon and the silicon polishing slurry is apolycrystalline silicon polishing slurry.
 24. The method of claim 22 ,wherein the silicon layer is amorphous silicon and the silicon polishingslurry is an amorphous silicon polishing slurry.
 25. The method of claim22 , wherein the silicon layer is single crystalline silicon and thesilicon polishing slurry is a single crystalline silicon polishingslurry.
 26. The method of claim 22 , further comprising polishing thesilicon layer at a second polishing pad with a second silicon-polishingslurry until the non-planar surface is exposed.
 27. The method of claim22 , further comprising stopping the supply of oxide-polishing slurry tothe polishing pad surface before the silicon-polishing slurry issupplied to the polishing pad.
 28. The method of claim 22 , wherein thesilicon-polishing slurry is supplied to the polishing pad after thesubstrate has been in contact with the polishing pad for less than aboutten seconds.
 29. The method of claim 22 , wherein the substrate ischemical mechanically polished with the oxide-polishing slurry for lessthan about ten seconds.
 30. The method of claim 29 , wherein thesubstrate is chemical mechanically polished with the oxide-polishingslurry for about three to five seconds.
 31. A method of polishing asilicon layer on a substrate, comprising: (a) chemical mechanicalpolishing the silicon layer with an oxide-polishing slurry to remove anative oxide layer; (b) chemical mechanical polishing the silicon layerwith a silicon-polishing slurry until the silicon layer is substantiallyplanarized; and (c) stopping step (b) before the silicon layer issubstantially planarized.
 32. A method of polishing a silicon layer on asubstrate, comprising: (a) chemical mechanical polishing the siliconlayer with an oxide-polishing slurry to remove a native oxide layer; (b)chemical mechanical polishing the silicon layer with a silicon-polishingslurry until an underlying non-planar surface is exposed; and (c)stopping step (b) before the underlying non-planar surface is exposed.